The Snåsa Mega-lens, west-central Scandinavian Caledonides

Abstract

The northernmost closure of the Trondheim depression features a tectonic mega-lens composed of five principal units of the Swedish (Jämtland) and Norwegian (Tröndelag) allochthons. These units are the Offerdal, Särv, Seve, Gula and Stören Nappes. The Köli Nappe of the Swedish allochthon appears to have pinched out completely. The basement-cover contact has been traced westwards into the Western Gneiss Region. The entire tcctono-stratigraphy was folded by the major Snåsa Synform and an associated antiform-synform pair. Folding was gentle in the eastern limb of the Snåsa Synform, and tight at the border to the gneiss region. Late high-angle faulting accentuated the fold geometry. The mega-lens folded by these folds provides further evidence of the pinch-and-swell tectonics of the Scandian (Late Silurian to Early Devonian) nappe displacement. Although sometimes less than 50 m thick, the Seve and Gula Nappes persist throughout the tight synforms. Such continuity and regular tectono-stratigraphical behaviour of thin, discrete nappes suggest that the nappes travelled together in composite units subjected to extreme stretching. Basal thrusts are zones of intense attenuation and do not truncate stratigraphical sections. Early, flattened cylindrical mesofolds with axial trends around N-S, and non-cylindrical mesofolds with similar average trends occur throughout the basement-cover sequence. Superimposed on these early folds are linear structures parallel to the ESE direction of nappe movement. These include folds, sheath folds, mineral lineations and quartz rodding. Transverse folds are common in the quartz segregations of the basal thrust and occur in the basement-cover contacts of the Western Gneiss Region. The Gula Nappe preserves evidence of a pre- or early Scandian metamorphic event of medium amphibolite facies. Resting on amphibolite facies and, further down, also greenschist facies rocks, the nappe represents an inverted metamorphic gradient. The core of the tight Flåtjern Synform is occupied by epidote amphibolite facies rocks of the Stören Nappe. Adjacent on both sides occur decidedly higher-grade schists of the Gula Nappe. Post-thrusting metamorphism above greenschist facies grade is conspicuous in zones of concentrated strain. Except for felsitic and calcareous lenses, the stretching lineation is penetrative in the upper, more strongly metamorphosed nappes. The lower nappes (i.e. Särv, Offerdal, porphyry-granite nappe) may, in contrast, exhibit primary sedimentary and igneous textures and were on the whole less ductile during the Scandian nappe displacement. Such contrasts of metamorphism and ductile deformation in the cover militate against orogenic models which sec this part of the Western Gneiss Region as the deep and central zone of the orogen. With regard to post-thrusting tectono-metamorphism (‘Caledonization’) of basement and cover, the Snåsa area represents a zone of remarkably rapid change.

Key Words:

• Nappes
• thrusts
• basement-cover relations
• mega-lens
• pinch-and-swell tectonics
• ferro-tschermakite
• sulphide mineralization
• Jämtland Supergroup
• Offerdal Nappe
• Särv Nappe
• Seve Nappe
• Gula Nappe
• Støren Nappe
• Grong-Olden Culmination
• Tømmerås Window
• Western Gneiss Region
• Snåsa Synform
• Caledonides
• Scandes
• Trondheim region
• Trøndelag
• west-central Norway
• N6400 N6430 E1200 E1245